Muscular system

• Myology


  The scientific study of muscles
• Cardiac muscle tissue


  Striated, involuntary, has single centrally located nucleus, cardiac muscle fibres are connected by intercalated discs, depends on aerobic cellular respiration to generate ATP, the intercalated disc contains gap junctions, adhering junctions and desmosomes
• Muscle types


  • Skeletal
  • Cardiac
  • Smooth
• Location of the cardiac muscle

  Cardiac muscle tissue, also known as myocardium, is a structurally and functionally unique subtype of muscle tissue located in the heart, it is capable of strong, continuous, and rhythmic contractions that are automatically generated, the contractility can be altered by the autonomic nervous system and hormones, it has high metabolic, energy, and vascular demands
• Skeletal muscle


  • Striated
  • Voluntary
• Intercalated discs


  • Cardiac myocytes are joined together via intercalated discs, which coincide with Z lines, contain three types of cell junctions: Adherens junctions- responsible for connecting the ends of the myocytes together to form a fibre, Desmosomes- prevent the separation of myocytes during contractions by anchoring intermediate filaments, Gap junctions- provide pathways for ions to pass between adjacent cells, resulting in the propagation of excitation and subsequent contraction
• Cardiac muscle


  • Striated
  • Involuntary
• Differences in contraction of skeletal and cardiac muscle fibres

  The depolarization of the sarcoplasm lasts longer in cardiac tissue, calcium channels are also present in the walls of the T tubule system, rather than being limited to the sarcoplasmic reticulum, the released calcium ions bind to calcium sensitive channels in the sarcoplasmic reticulum, which results in a large and fast release of further calcium ions required for contraction
• Smooth muscle


  • Non-striated
  • Involuntary
• Smooth muscle tissue


  Involuntary, nonstriated, in addition to thin and thick filaments, SMT contain intermediate filaments & dense bodies, location: the walls of hollow viscera and of blood vessels, large airways to the lungs, arrector pili muscles, and the eye, and reproductive system tracts, myofibroblasts are a special type of smooth muscle cell that produce connective tissue proteins such as collagen and elastin, are found in alveolar septa of the lung and scar tissue
• Functions of muscular tissue

  • Producing body movements
  • Stabilizing body positions
  • Regulating organ volume
  • Moving substances within the body
  • Releasing heat
• Smooth muscle structure


  • A dense body is analogous to the Z-discs of skeletal muscle, anchoring the thin filaments in position, intermediate filaments involved in pulling the sarcolemma toward the fibre's middle, shortening it in the process
• Epimysium


  Covering an entire muscle
• Smooth muscle tissue


  The duration of contraction and relaxation is longer in smooth muscle than in skeletal muscle, smooth muscle tone is a state of continuous partial contraction of smooth muscle tissue, smooth muscle fibers can be stretched considerably and still retain the ability to contract, controlled with signals from the autonomous nervous system, such as nerve impulses, hormones, and other chemicals released by specialized organs
• Perimysium


  Covering fascicles
• Structure of the smooth muscles

  There is no a special protein on actin which prevents myosin from binding, actin and myosin are constantly binding, the level of calcium controls the amount of ATP available to myosin, when the stimulus is removed, the cells do not relax right away, myosin continues to bind to actin and crawl along the filaments until the level of calcium falls
• Endomysium


  Covering individual muscle fibres
• Motor units of smooth muscle

  A series of axon-like swelling, called varicosities or "boutons," from autonomic neurons form motor units through the smooth muscle
• Tendons


  Extensions of connective tissue beyond muscle fibres that attach the muscle to bone
• Smooth muscles in the digestive system

  Sheets of smooth muscle tissue in the DS react to swallow, tension is applied to one side of the sheet, the cells on that side contract in reaction, a wave begins to propagate itself down the digestive tract, this phenomena is known as peristalsis, and is responsible for moving food through the many twists and turns of the gut
• Skeletal muscles are well supplied with nerves and blood vessels, which provide nutrients and oxygen for contraction
• Aging and muscular tissue


  Beginning at about 30 years of age, humans undergo a slow, progressive loss of skeletal muscle mass that is replaced largely by fibrous connective tissue and adipose tissue, in part, this decline is due to decreased levels of physical activity, aging also results in a decrease in muscle strength, slower muscle reflexes, and loss of flexibility
• Muscle fasciculi


  Groups of skeletal muscle fibres
• How skeletal muscles produce movement

  Skeletal muscles are not attached directly to bones, they produce movements by pulling on tendons, which, in turn, pull on bones, the attachment to the stationary bone is the origin, the attachment to the movable bone is the insertion, the prime mover (agonist) produces the desired action, the antagonist produces an opposite action, the synergist assists the prime mover by reducing unnecessary movement, the fixator stabilizes the origin of the prime mover so that it can act more efficiently
• Skeletal muscle


  Many muscle fasciculi joined together
• Characteristics used to name skeletal muscles

  • Direction of fibers
  • Location
  • Size
  • Number of origins
  • Shape
  • Origin and insertion
  • Action
• Skeletal muscle fibre


  A multinucleated cell that acts as a single contractile unit and can undergo contraction and relaxation independent of the rest of fibres
• Characteristics used to name skeletal muscles - Direction

  • Rectus (parallel to midline)
  • Transverse (perpendicular to midline)
  • Oblique (diagonal to midline)
• Sarcolemma


  The plasma membrane that encloses the entire skeletal muscle fibre
• Characteristics used to name skeletal muscles - Action

  • Flexor (decreases joint angle)
  • Extensor (increases joint angle)
  • Abductor (moves bone away from midline)
  • Adductor (moves bone closer to midline)
  • Levator (produces superior movement)
  • Depressor (produces inferior movement)
  • Supinator (turns palm anteriorly)
  • Pronator (turns palm posteriorly)
  • Sphincter (decreases size of opening)
  • Tensor (makes a body part rigid)
• Transverse tubules (T-tubules)


  Tunnels that extend from the surface toward the center of each muscle fibre, connected with sarcoplasmic reticulum on either side to form a triad
• Characteristics used to name skeletal muscles - Number of origins

  • Biceps (two origins)
  • Triceps (three origins)
  • Quadriceps (four origins)
• Principal skeletal muscles


  The names of most of the nearly 700 skeletal muscles are based on specific characteristics including direction of fibers, location, size, number of origins, shape, origin and insertion, and action
• Sarcoplasm


  The fluid-filled space within a skeletal muscle fibre that contains multiple nuclei, many mitochondria, myoglobin, and sarcoplasmic reticulum
• The muscular system contributes to homeostasis
• Myofibrils


  Structures within each muscle fibre that contain thin and thick filaments
• Disorders of the muscular system

  • Myasthenia Gravis (autoimmune disease that causes chronic, progressive damage of the neuromuscular junction)
  • Muscular Dystrophy (inherited muscle‐destroying disease that cause progressive degeneration of skeletal muscle fibres, the mutated on the X chromosome)
  • Fibromyalgia (painful, nonarticular rheumatic disorder that usually appears between the ages of 25 and 50)
• Sarcomere


  The functional unit of striated muscle fibres, separated by Z discs
• Thick filaments


  Composed of myosin
• Thin filaments


  Composed of actin, tropomyosin, and troponin